1. Field of the Invention
The present invention relates to a multi-layer solid golf ball comprising a core, an intermediate layer, and a cover, and more particularly to a golf ball in which, at a boundary between a core and an intermediate layer or at a boundary between the intermediate layer and a cover, convex ribs and columnar projections are formed on one of the adjacent layers (the core or the intermediate layer) such that the convex ribs and the columnar projections intrude into the other layer (the intermediate layer or the cover).
2. Description of the Related Art
Various techniques have been studied and proposed for increasing travel distance of a golf ball and providing a player with an excellent feel upon hitting the golf ball (hereinafter may be called xe2x80x9chit feelxe2x80x9d). Especially in a golf ball having a solid core and a cover, the hardness and size (diameter and thickness) of the core and the hardness and size of the cover are adjusted for such purposes.
For example, U.S. Pat. No. 5,439,227 discloses a three-piece golf ball which has a solid core, an inner cover, and an outer cover and in which the outer cover is made harder than the inner cover. Also, U.S. Pat. No. 5,490,674 discloses a three-piece golf ball which has inner and outer solid cores covered with a cover and in which the inner solid core is made harder than the outer solid core.
In the above-described golf balls, the boundary surface of each layer is generally a smooth spherical surface having neither projections nor depressions. However, U.S. Pat. Nos. 2,376,085 and 5,692,973 disclose a golf ball which has on its solid core a plurality of projections for preventing eccentricity of the solid core, which eccentricity could otherwise arise when a cover is formed around the core through injection molding.
The projections on the solid core of the above-described golf ball are designed to substitute support pins used in an injection molding process, and the effect obtained by the shape of the support-pin-shaped projections is not utilized to improve the performance of the golf ball. In other words, the inventions of U.S. Pat. Nos. 2,376,085 and 5,692,973 relate to a technique for preventing eccentricity of the solid core and preventing mixture of a different material into the cover. According to the technique, by employment of the same material as used for the cover, projections are formed on the core surface such that the cover has a uniform thickness, and the projections and the cover are thus united. As described above, the projections are not designed to improve the performance of the golf ball.
Also, Japanese Patent Application Laid-Open (kokai) No. 9-285565 discloses a two-piece golf ball which has projections and depressions between a solid core and a cover, between two adjacent layers of a multi-layer solid core, or between two adjacent layers of a multi-layer cover. The two-piece golf ball provides a player with different hit feels, depending on the direction of an external force acting on the golf ball during hitting.
The two-piece golf ball has improved in terms of hit feel provided to a player. However, the travel performance and durability are not satisfactory, and there is room for further improvement.
In view of the foregoing, an object of the present invention is to provide a golf ball which has an internal structure such that, at a boundary between a core and an intermediate layer or at a boundary between the intermediate layer and a cover, convex ribs and columnar projections are formed on one of adjacent layers (the core or the intermediate layer) such that the convex ribs and the columnar projections intrude into the other layer (the intermediate layer or the cover), which has an improved travel performance and controllability, as compared with a conventional golf ball, and which provides a player with an improved hit feel as compared with a conventional golf ball.
To achieve the above object, the present inventors have conducted earnest studies, taking notice that when the effect of the configuration at a boundary between the layers of a golf ball; i.e. the cross-sectional, two-dimensional moment of a member that constitutes each of the convex ribs is increased, the bending strength of the member can be increased with no corresponding increase in hardness.
As a result, the present inventors found the following with regard to a multi-layer solid golf ball in which convex ribs are formed at a boundary between adjacent first and second layers; i.e., between a core and an intermediate layer or between the intermediate layer and a cover. When convex ribs are formed in a network pattern on the first layer having a relatively high hardness such that the convex ribs intrude into the second layer having a hardness less than that of the first layer, and columnar projections are formed on the first layer at positions corresponding to the nodes of the network pattern such that the columnar projections intrude into the second layer deeper than the convex ribs, due to the effect of the shapes of the convex ribs and the columnar projections, the bending strength of the member constituting the network-shaped convex rib increases, because the member is supported by the columnar projections at positions corresponding to the nodes of the network pattern. As a result, when the golf ball is hit at a relatively high head speed by use of a driver or a like club, the degree of backspin of the golf ball decreases and the travel distance increases accordingly, whereas when the golf ball is hit at a relatively low head speed by use of a short iron or a like club, the hardness of the member does not exceed a level of hardness in conventional golf balls, yielding excellent controllability and providing soft feel.
The present invention was accomplished on the basis of the above-described findings, and provides a golf ball which comprises a core, an intermediate layer, and a cover, wherein at a boundary between the core and the intermediate layer or at a boundary between the intermediate layer and the cover, convex ribs are arranged in a network pattern on a first layer having a relatively high hardness such that the convex ribs intrude into a second layer having a hardness less than that of the first layer, the first and second layers being adjacent to each other; and columnar projections are formed on the convex ribs at positions corresponding to nodes of the network pattern such that the columnar projections intrude into the second layer deeper than the convex ribs.
The golf ball according to the present invention has the following advantageous features:
(i) When the golf ball is hit at a relatively high head speed by use of a driver or a like club, the degree of backspin of the golf ball decreases, and the travel distance increases accordingly.
(ii) When the golf ball is hit at a relatively low head speed by use of a short iron or a like club, the degree of backspin increases, so that excellent controllably is maintained.
(iii) A player is provided with a soft feel when hitting the golf ball with a driver, and is provided with a firm and solid feel when hitting the golf ball with a short iron.
(iv) In the case where the second layer is injection-molded around the first layer having the convex ribs, or in the case where the second layer is injection molded to have on its outer surface depressions corresponding to the convex ribs and the columnar projections, passages through which resin flows are secured within the cavity of a mold during the injection molding, so that the first layer having the convex ribs and the columnar projections and the second layer into which the convex ribs and the columnar projections intrude can be molded properly, imparting improved symmetry to the golf ball.